The invention relates to a method of measuring spectral attenuation of an optical waveguide fiber using an optical time domain reflectometer (OTDR). In particular, the spectral attenuation of the waveguide, over a selected wavelength range, is predicted from OTDR measurements at least at three selected wavelengths.
The unrepeatered distance over which an optical waveguide can transport information depends directly on the waveguide attenuation. Further, as the information rate has increased, the practice of increasing waveguide capacity by using wavelength division multiplexing has become common.
Thus the need has arisen to have accurate knowledge of the waveguide attenuation over a range of wavelengths defining an operating window and over a number of operating windows. That is, there is a need for an accurate measurement of waveguide attenuation over a wide range of wavelengths. A typical wavelength range is 1200 nm to 1600 nm, which includes an operating window around 1300 nm and another around 1550 nm.
However waveguide uses have extended over a wavelength range from about 700 nm to about 2000 nm. To obtain spectral attenuation for this wider range, which includes an operating window centered at about 850 nm, a set of lasers having appropriate center wavelengths which span a selected wavelength range must be chosen.
Waveguide attenuation measurements over a range of wavelengths, i.e., spectral attenuation, may be done using a laser at each wavelength of interest and measuring light power loss versus fiber length. A well known attenuation measurement method includes measurement of a reference fiber or a short cut back fiber. A ratio of power transmitted through the full length of a waveguide to power in the reference or short length fiber yields the power ratio which defines attenuation. Time consuming multiple measurements must be made. Furthermore, the number of lasers required makes the spectral attenuation measurement costly and more difficult to maintain. A preferred alternative to multiple laser sources is the use of a monochromator. A typical grating monochromator is capable of providing monochromatic light in steps of a few nanometers over a range of about 50% to 180% of the blaze wavelength. For example, a grating monochromator having a blaze wavelength of 1000 nm can in theory provide spectral measurements over a range from about 500 nm to about 1800 nm, although power output usually falls off near the ends of the operating range. A monochromator is costly and must be maintained in calibration.
A number of spectral measurement systems must be used to keep pace with the manufacturing rate. Hence, initial cost and maintenance cost is high.
Thus there is a need for a spectral attenuation measurement which is lower in maintenance cost, which shortens measurement time, reduces the number of measuring systems required and which does not require a monochromator or a large number of lasers.